Self-cycled photo-Fenton-like system based on an artificial leaf with a solar-to-H2O2 conversion efficiency of 1.46%

Millions of families around the world remain vulnerable to water scarcity and have no access to drinking water. Advanced oxidation processes (AOPs) are an effective way towards water purification with qualified reactive oxygen species (ROSs) while are impeded by the high-cost and tedious process in either input of consumable reagent, production of ROSs, and the pre-treatment of supporting electrolyte. Herein, we couple solar light-assisted H2O2 production from water and photo-Fenton-like reactions into a self-cyclable system by using an artificial leaf, achieving an unassisted H2O2 production rate of 0.77 μmol/(min·cm2) under 1 Sun AM 1.5 illumination. Furthermore, a large (70 cm2) artificial leaf was used for an unassisted solar-driven bicarbonate-activated hydrogen peroxide (BAP) system with recycled catalysts for real-time wastewater purification with requirements for only water, oxygen and sunlight. This demonstration highlights the feasibility and scalability of photoelectrochemical technology for decentralized environmental governance applications from laboratory benchtops to industry.

• On page 8, line 157, there is no end bracket next to ' Supplementary Fig. 6' • From method section, line 399, authors synthesized WO3 at 110˚C for 40 min, but from Table S1, optimized WO3 is synthesized at 110˚C for 60 min. Make them clear.
• From Fig. 5c, 5d and 6c, change time unit from capital 'S' to small 's' Reviewer #3 (Remarks to the Author): In this manuscript, the author designs a self-cycled photo-Fenton-like system for real-time wastewater purification. This is an interesting study. However, in terms of the mechanistic discussion, the authors do not consider the role of reactive species other than hydroxyl radicals at all. Therefor I don't think it can be published. 1. It is well known that the removal of pollutants may be the result of the combined action of multiple active species. However, only the role of hydroxyl radicals is explored in the manuscript, and the possible roles of other reactive species such as 1O2 and O2-are not considered..

2.
As mentioned before, the authors constructed a relatively complex system including catalysts, O2, HCO3-, Mn(II) and Mn(III), etc. For such a system, the contribution of multiple active species to pollutant degradation and the possible mutual transformation between them should be systematically considered. In fact, there may be 1O2 and O2-intermediates for the formation of H2O2. Therefore, I do not think it is reliable to attribute removal of pollutants entirely to hydroxyl radical.
3. The author proposes that the generation process of H2O2 is O2 + 2H2O → 2H2O2. If the external energy output is sufficient to drive this reaction, holes may directly oxidize H2O/OH-to •OH. Did the author consider this process?
Reviewer #1: In this research work, the authors have constructed a self-cyclable photo-Fenton-like system with an artificial leaf for the production of H 2 O 2 from water. The system was also scaled up to 70 cm 2 to be utilized in an unassisted solar-driven bicarbonate-activated hydrogen peroxide system, which is inspiring for future breakthroughs for sustainable environmental applications. Although the authors have conducted the experiment with excellent care, there is a lack of fundamental knowledge and insights via computational simulations. It is crucial to integrate experiment and computation for aiming for a top-tier journal to reflect the nature of fundamental and application-driven processes. In view of the extremely high requirement, impacts, and novelty of Nature Communications, this work should be rejected without further review.
This work can be considered in the sister journal (e.g. Communications Chemistry) after appropriate revisions from the authors.
Our Response: We thank you very much for your evaluation of our work. The questions and suggestions raised by you are extremely important and helpful, which make us deep thought, thereby improving the quality of our work in the revision. As the reviewer said, the main target of this work is to construct a self-cycled photo-Fenton-like system with high solar-to-H 2 O 2 (STH) efficiency. The kinetic optimization of the photoanode and cathode is the key to achieve a high STH efficiency towards pollutant degradation application, and the modified approaches are extended researches based on our previous work which carefully explored the activity of the catalyst with systematically theoretical simulation. The related DFT calculation will be introduced in the following parts and we have done a series of investigations on morphology, chemical composition, valance state, catalysis performance et.al. The presentation video is also newly included. The main contribution of our work is further strengthened, and we believe the quality of the paper is significantly improved.
Our Response: Thanks for your wise suggestion. In our work, the PEC measurements in a three-electrode system are used to demonstrate the operating possibility of the artificial 1#1: The authors should link how their PEC system correlates to an artificial leaf. leaf as well as achievable theoretical current density (efficiency). For actual H 2 O 2 measurements, a two-electrode PEC system was conducted without using applied bias ( Fig. 6c in the manuscript). The operating conditions in the two-electrode PEC system are the same as that in the artificial leaf, as both are light-driven reactions. We have already claimed this point in our manuscript.
Our Response: Thanks for your kind advice. We have briefly added the description about limitations in the conclusion, as follows: "Notably, some improvements in this PEC-driven wastewater treatment system are needed for industrial-scale applications, including the enhanced solar utilization efficiency, continuous oxygen supplement, and tuning electrolyte components. Therefore, additional discussion for addressing the above issues was carried out to identify the limitations, which are expected to be addressed to achieve further breakthroughs." and more detailed discussions have been added in our supplementary information as follows: 1. The solar utilization efficiency. One of the significant goals of our system is to build a sustainable AOPs system with self-generated H 2 O 2 based on the PEC technology. A Higher H 2 O 2 generation rate is highly expected to accelerate the rate of AOPs due to the rapid consumption of H 2 O 2 . Based on that concept, kinetic optimization of photoanode and cathode is carefully and deliberately investigated to raise the H 2 O 2 generation rate, namely, STH efficiency, as much as possible. Even though, we expect that the STH efficiency can be substantially raised by introducing the state-of-art photovoltaic (PV) in the PEC system because the only part range of solar energy is absorbed (<510 nm) due to the relatively large band gap of the SnO 2-x /BiVO 4 /WO 3 photoanode (~2.45 eV). Therefore, the combination of a PV cell with a complementary absorption range (e.g. ~1.2 eV) could fulfill a broader absorption range with an extra driving force, which could lead to a higher working current density of the tandem PEC system and the artificial leaf.
1#2: Include a brief description of the limitations to be addressed in the conclusion.
The authors can also mention that a more detailed discussion can be found in the supplementary information.
According to previous theoretical research (Ref. R1), the unassisted working current density is expected to be raised by 3-4 times when a qualified perovskite cell was combined.
2. Supplement of oxygen. In this work, the oxygen supplement for the 2e-ORR in the cathode relies on the aeration from a high-pressure oxygen bottle during the whole reaction process. Therefore, it would make large progress if the oxygen/air could naturally diffuse into the cathode without external compression. Recently, a few researchers reported the concept of a deliberately designed ORR cathode with a superhydrophobic interface that allows the rapid and natural transport of O 2 from the atmosphere to the cathode through the backside of the electrode (Ref. R2, 3). Our work, on the other hand, takes care of both conductivity and oxygen diffusion efficiency by creating quasi-nanoarray aerophilicity areas only on the electrocatalysts coating side (front side). Therefore, it is reasonable to believe that an "asymmetric sandwich" electrode with modified electrocatalysts coating the front side and oxygen natural diffusion backside could avoid aeration energy consumption with improved electrocatalysis performance at the same time, which is believed to substantially enhance the feasibility of our device application.
3. Electrolyte in the natural environment. The environmental benignity is one of the most prospecting merits of our device because the reactors and the products involved are only oxygen, water and hydrogen peroxide and the electrolyte is bicarbonate aqueous solution, which is ubiquitous in groundwater, showing strong competitiveness sustainably compared to the traditional AOPs. However, the relatively high electrolyte concentration (0.4 M HCO 3 − ) is necessary for high H 2 O 2 generation current density in the PEC system. Therefore, we expect a potential electrolyte with a lower concentration for high H 2 O 2 generation current density in further research.
We hope that this discussion of the limitation and potential approaches will inspire further research toward further exploration of effective and sustainable wastewater treatment as well as efficient solar-fuel production/utilization devices. This part is included in the supplementary discussion.
Our Response: Thanks for your kind advice. We are sorry for the unclear description of the methodology in the previous version. We have detailedly described our methodology in our revised manuscript, which is believed to be easily followed by interested readers.

PEC Measurements.
There are two systems (three-electrode system and two-electrode system) for the PEC measurements to investigate the activity of photoanode. Specifically, the three-electrode system was carried out using photoanode as working electrode, saturated Ag/AgCl as reference electrode and carbon electrode or designed cathode as counter electrode, among which the designed cathode was taken as counter electrode only in the tandem device. The two-electrode system was carried out to evaluate the tandem device using photoanode as working electrode and cathode as counter electrode. All All current densities were normalized to the geometrical area of the photoanode. An 1#3: The methodology should be more detailed. Provide details on the reaction medium used for water and bicarbonate-activated H 2 O 2 production, as well as the reactor. How long is O 2 purged through the electrolyte? The present methodology is not well described and sound.  1#4: The first-principles DFT calculations are necessary to bridge experiment and calculation. This is vital to be included in the manuscript to unravel the reaction pathway. The density of states (DOS) should also be studied.  In this work, we presented a rationally and ingeniously designed artificial leaf by using the above two materials and further developed a self-cycled photo-Fenton-like system by introducing Mn 2+ /Mn 4+ redox, which achieved a world record of solar to H 2 O 2 for pollution degradation. The improved strategy and novelty can be summarized as follows: 1. By using SnO 2-x /BiVO 4 /WO 3 instead of SnO 2-x /BiVO 4 , the onset potential was cathodically shifted to successfully couple with a cathodic reaction. This Table is included as supplementary Table 1. The SEM image of photoanode after long-term stability testing shows the morphology of SnO 2-x /BiVO 4 /WO 3 is well-maintained (Fig. R3a). Meantime, HR-TEM also demonstrates a conformally and stable SnO 2-x overlayer on BiVO 4 (Fig. R3b).
Moreover, EPR measurement evidenced the maintenance of the oxygen vacancies of the SnO 2-x layer after stability tests (Fig. R3c), which indicated that the passivation layer of SnO 2-x seems to be responsible for the good stability of SnO 2-x /BiVO 4 /WO 3 . peaks were determined to be stable during PEC testing (Fig. R4b). On the other hand, the Mo-SACs-mrG-GDE also shows good stability with wellmaintained singly dispersed atoms (Fig. R5a) and quasi-nanoarray aerophilicity areas (Fig. R5b). The modified oxygen-diffusion-benefit structure is supposed to contribute to good stability since the insufficient oxygen supplement during large current density would damage the catalysts.   with a production rate of 5.17 mol min-1 is achieved by this system. Furthermore, the successful application was found when such a system was applied for the degradation of 4-nitrophenol in one-pot self-cycled wastewater treatment. Overall, the manuscript introduces a novel concept for the dual application of artificial photosynthesis of H 2 O 2 and greener wastewater treatment. The manuscript is a well-written interesting paper and is publishable in "nature communications" after the modifications considering comments below.
Our Response: We thank you very much for your positive evaluation of our work. As will be shown below, we have polished our manuscript towards higher quality through significant modifications based on your suggestions.
Our Response: Thanks for your kind advice. We have added the references for the band gap values in the revised manuscript.
Our Response: Thanks for your wise advice. It is wise to slow the scan rates during the LSV measurements to minimize the impact of the non-Faradic current. Therefore, we have retested the LSV of both cathode and photoanode with various scanning rates of 2, 5 and10 mV/s (original scan rate). As shown in Fig. R6, a slightly higher current density is whereas the current was reduced to 1.56 mA cm -2 in Fig. 5b. The authors need to explain this difference and need to measure LSV at lower scan rates too to exactly determine the intersection current.
observed at a higher scanning rate. In our revised manuscript, we have unified the scanning rate to 2 mV/s, which achieves a theoretical intersection current of 1.64 mA/cm 2 (0.61 V vs. RHE) that is closed to the value measured in the two-electrode system. The reason for the slight difference between theoretical value and the actual bias-free current might be attributed to the polarization that occurs at the counter electrode (Ref. R8) in the two-electrode system compared to the three-electrode system. 2#3: The authors need to add the error bars in Fig. 3g, Fig. 4b, Fig. 4e, Fig. 5d, and Fig. 6d to increase the reliability of data.
Our Response: Thanks for your kind advice. We are sorry for these mistakes caused by our carelessness. All the format and statements are carefully corrected and note that The geometric area of artificial leaf used in Fig. 6 is 7 cm 2 ).
• Citation of Fig. in line 88.
• Line 140, the subscript of production rate.
• Numerical value of H2O2 selectivity in line 222.
• On page 8, line 157, there is no end bracket next to " Supplementary Fig. 6" • From method section, line 399, authors synthesized WO3 at 110˚C for 40 min, but from Table S1, optimized WO3 is synthesized at 110˚C for 60 min. Make them clear.
• From Fig. 5c, 5d and 6c, change time unit from capital "S" to small "s" Reviewer #3: In this manuscript, the author designs a self-cycled photo-Fenton-like system for real-time wastewater purification. This is an interesting study. However, in terms of the mechanistic discussion, the authors do not consider the role of reactive species other than hydroxyl radicals at all. Therefore I don"t think it can be published.    The authors have thoroughly revised the manuscript based on the comments raised by all reviewers. The novelty of the work is high. The manuscript reaches the requirement for consideration in Nature Communications. I am happy with the present revision.
Reviewer #2 (Remarks to the Author): Authors have addressed the comments, although authors forgot to update the error bars in the following comment.
Revised manuscript is now appropriate to be published in this journal.
1. The authors should add additional experiments data to add the error bar in Fig. 3g, Fig. 4b, Fig. 4e, Fig. 5d, and Fig. 6d.
Reviewer #3 (Remarks to the Author): I again reviewed the revised manuscript with the author's response. I think their reply could not answer the problems noted.
1. The authors only investigated the signal intensities of 1O2 and O2-in different systems through EPR, which is far from enough to explain the reaction mechanism. The authors did not even study the contribution of the generated 1O2 and O2-converted from H2O2 or O2 to pollutant removal, which is the most basic aspect. 3. For 1#4 proposed by reviewer 1, the author also did not answer directly and made corresponding revisions. Reviewer 1 proposed that the content of DOS should be added. However, the author did not perform corresponding theoretical calculations, but cited "seemingly similar" literature results.
The corresponding theoretical calculations should be carried out for different reaction systems, otherwise it is meaningless. Moreover, the results of the literature cited by the authors were simulations of reaction pathway, which were completely different from the DOS calculations proposed by reviewer 1.
3. Before the reaction, there are only catalysts, O2, HCO3-and Mn(II) in the reaction system, and there is no H2O2, because H2O2 is generated after the reaction starts. However, the authors did not consider the case where only HCO3-and Mn(II) exist when using EPR to study the active species in the reaction, but the case where HCO3-, Mn(II) and H2O2 exist. Therefore, I must say that the EPR done is unconvincing to study the mechanism of the current reaction system. 4. The author believes that ROS such as 1O2 and O2-are converted from H2O2 only through EPR experiments, which is unconvincing. It is well known that O2 can be converted to O2-or 1O2, and O2-can also be converted to 1O2. Moreover, ROS directly converted from O2 may also directly participate in the degradation of pollutants before generating H2O2, which is also a key point that the authors did not consider. Until this critical point is verified, the author's proposed "Self-cycled photo-Fenton-like system" is an unreliable conclusion.
5. The specific roles of Mn(II) and HCO3-on the generation and interconversion of ROS are also very important. In some similar studies, the researchers only added one substance to the traditional reaction system, and used at least four different experimental methods to systematically explore the reaction mechanism (Angewandte Chemie, 133(6), 2939-2944; Environ. Sci. Technol. 2019, 53, 9725−9733). The reaction system proposed by the author is obviously more complex, and it is more necessary to systematically study the reaction mechanism, rather than simply obtaining an unreliable conclusion through EPR.
6. For 3#3, the reaction system of this work is quite different from that of ref 4, and the previous conclusions do not apply to this work.
In general, the author did not directly answer or directly ignored the questions raised by me and other reviewers, and this work has very serious problems in the reaction mechanism. Therefore, I do not think this work is suitable for publication.
Dear Reviewers: 4 We sincerely thank for your valuable comments. In our previous manuscript, we focused 5 on materials, device configuration and performances of our electrochemical devices. So, 6 Our description did not satisfy your question. In this revision, we have particularly focused 7 on the reaction mechanism issues during the ROS generation which is raised by reviewer  Fig. 3g,   Fig. 4b, Fig. 4e, Fig. 5d, and Fig. 6d.
Reviewer #3: I again reviewed the revised manuscript with the author's response. I think 32 their reply could not answer the problems noted.  over PTFE@Mo-SACs/mrG-GDE were performed. As shown in Fig. R2a and b were also observed in the anode part ( Fig. R2c and d)    3#3: For 1#4 proposed by reviewer 1, the author also did not answer directly and made corresponding revisions. Reviewer 1 proposed that the content of DOS should be added.
However, the author did not perform corresponding theoretical calculations, but cited "seemingly similar" literature results. The corresponding theoretical calculations should be carried out for different reaction systems, otherwise it is meaningless. Moreover, the results of the literature cited by the authors were simulations of reaction pathway, which were completely different from the DOS calculations proposed by reviewer 1. 3#4: Before the reaction, there are only catalysts, O2, HCO3and Mn(II) in the reaction system, and there is no H2O2, because H2O2 is generated after the reaction starts.
However, the authors did not consider the case where only HCO3and Mn(II) exist when using EPR to study the active species in the reaction, but the case where HCO3 -, Mn(II) and H2O2 exist. Therefore, I must say that the EPR done is unconvincing to study the mechanism of the current reaction system. product of O2 reduction in the cathode is H2O2 (Fig. 4a). Above all, we believed that the in the self-cycled photo-like system. As shown in Fig. R4a, barely degradation effect is 138 observed in the absence of Mn 2+ , even if H2O2 is effectively generated in the reaction 139 system (via 2e-ORR in cathode) (Fig. R4b), which indicates KPi cannot activate H2O2 into 140 ROS as the HCO3 − does (Fig. 6d and Fig. S1). Furthermore, the degradation rate in KPi 141 electrolyte slightly raised to 5.4% at 40 min when Mn 2+ is added, which is still much lower 142 3#5: The author believes that ROS such as 1O2 and O2are converted from H2O2 only through EPR experiments, which is unconvincing. It is well known that O2 can be converted to O2or 1O2, and O2can also be converted to 1O2. Moreover, ROS directly converted from O2 may also directly participate in the degradation of pollutants before generating H2O2, which is also a key point that the authors did not consider. Until this critical point is verified, the author's proposed "Self-cycled photo-Fenton-like system" is an unreliable conclusion.
3#6: The specific roles of Mn(II) and HCO3on the generation and interconversion of ROS are also very important. In some similar studies, the researchers only added one substance to the traditional reaction system, and used at least four different experimental methods to systematically explore the reaction mechanism (Angewandte Chemie, 133(6), 2939-2944; Environ. Sci. Technol. 2019, 53, 9725−9733). The reaction system proposed by the author is obviously more complex, and it is more necessary to systematically study the reaction mechanism, rather than simply obtaining an unreliable conclusion through EPR.

186
In the self-cycled system, the transformation of Mn Ⅳ species into Mn Ⅱ species would be 187 largely enhanced through eq(7) by the reduction of a cathode according to Fig. 1d  (3#3: The author proposes that the generation process of H2O2 is O2 + 2H2O → 2H2O2. If the external energy output is sufficient to drive this reaction, holes may directly oxidize H2O/OH-to •OH. Did the author consider this process?)